1) Field of Invention
The present invention generally relates to nano-indentation measuring instruments and more particularly to a measuring head for such an instrument as well as to its method of use, said head using active referencing of the surface of the sample to be measured.
2) Description of Related Art
The measuring method by instrumented indentation (Depth-Sensing Indentation or DSI) is one of the most used methods for determining certain mechanical properties of materials, such as for example the elastic modulus and hardness. This method consists of applying an increasing and subsequently a decreasing force on a sample via a tip or indentor, with a determined shape in order to investigate and to measure, continuously and simultaneously, the values of the load, of the load alleviation, respectively, applied onto the sample on the one hand, and the penetration depth of the indentor on the other hand. The involved forces may be extremely weak, typically of the order of a few micro-Newtons (μN), and the displacements of the indentor to be measured may be of the order of one nanometer (nm). One then speaks of nano-indentation.
A head for nano-indentation measurements should therefore be capable of applying a force on a sample via an indentor and of determining the applied force as well as the corresponding penetration of the indentor. Among the existing nano-indentation measuring instruments, the Nano Hardness Tester or NHT, may be cited, marketed by the applicant or further the instrument “Triboindenter®” of Hysitron Inc. The technology used for the measuring head of this latter instrument is moreover described in U.S. Pat. No. 5,553,486. The measuring heads of the prior art apply the desired force via an electrodynamic (electro-magnet) or electrostatic actuator and they have a displacement sensor (generally a capacitive sensor) but no force sensor. The force applied to the sample is then inferred from the force generated by the actuator, for example calculated from the current or voltage applied to the latter and from the stiffness of the spring strips supporting the indentor. This is therefore a method for indirect measurement of the applied force.
U.S. Pat. No. 4,852,397 describes a measuring instrument by nano-indentation, the measuring head of which includes an indentor, actuating means as well as detecting means of the penetration depth of the indentor in a sample by direct measurement.
The publication entitled “Development of a depth controlling nano-indentation tester with subnanometer depth and submicro-newton load resolutions”, published under the names of Atsushi Shimamoto and Kohichi Tanaka, in Volume 68(9) of September 1997 of the Review of Scientific Instruments, pp. 3494-3503, discloses another example of such a measuring apparatus. The latter comprises a measuring head, including an indentor and a displacement sensor, both submitted to the action of a single actuator, as well as means for measuring the load applied by the actuator, distinct from the measuring head. The displacement sensor described in this publication is of the optical fiber type and with it, direct optical measurement of the displacement of the measuring head may be carried out.
Generally, with the cited devices of the prior art, several problems arise, such as the following:
1) Thermal drift. A standard nano-indentation measurement lasts for about one minute. During this period, any change in temperature will result in a dimensional change of the mechanical components of the measuring instrument (thermal expansion or contraction). The problem is further worsened if the instrument uses an electrodynamic actuator which itself produces heat and this depends on the generated force. For example, in the case of a system without any reference, if a path of 30 cm is considered between the tip of the indentor and the surface of the sample, via the frame of the steel instrument (thermal expansion coefficient of steel equal to 10×10−6/° C.), a variation of 0.1° C. during the measuring period would lead to a depth measuring error of the order of 300 nm, which is disproportioned with regard to the penetration depths which themselves are frequently less than 100 nm. Present answers for minimizing thermal variations combine the use of costly thermostated enclosures with an action consisting of measuring the drift at a given instant and of applying a correction of this drift to the totality of the nano-indentation measurement. Such answers lead to very long cycle times (waiting for thermal stabilization in the enclosure) and are based on the arbitrary assumption that the thermal drift remains constant throughout the measurement.
2) Influence of the stiffness of the frame of the instrument and of the frame-sample connection. When a force is applied on the indentor, it not only causes a penetration of the indentor into the sample but also a deformation of the sample holder/instrument assembly proportional to the compliance (which means the capability of deforming under the effect of a stress) of this assembly. This deformation leads to overestimating the penetration depth and requires subsequent correction by subtraction of the estimated value of the compliance. A solution, already used by the applicant to counter this problem, consists of having the measuring head directly rest on the sample via a reference part and next measuring the penetration depth directly between this reference and the indentor. Any parasitic movement (whether from a thermal origin or because of the compliance of the frame) will thus be greatly attenuated. One of the problems in this case, is that the totality of the weight of the measuring head rests on the sample, which poses problems on soft materials or materials exhibiting creep because, in this case, solving a problem creates new problems.
3) Independence of the force and displacement sensors. As indicated earlier, the existing nano-indentation heads are equipped with a single actuator and a single sensor (displacement sensor). The absence of an independent force sensor poses two problems. First, the force is not directly measured but is estimated, which may be a source of error, and second, several very interesting loading modes—such as an indentation with a constant deformation rate for example—cannot be applied completely because they require a control which takes into account the penetration depth and the applied force simultaneously.